Product Focus

802.11n Bridging is Here

Developed for broadband operators looking to deliver reliable wireless broadband services, the ZoneFlex 7731 is the first smart Wi-Fi point-to-point backhaul system that delivers consistent performance at ranges up to 15 kilometers. Based on the 802.11n standard combined with a Ruckus’ smart directional antenna, the ZoneFlex 7731 supports up to 190 Mbps at 1.5 kilometers, and offers performance up to 50 Mbps at 10 km (LoS). One of the methods 802.11n uses to increase throughput is MIMO (multiple-in / multiple-out). MIMO requires multipath reflections, which are not common in point-to-point bridging scenarios. Therefore, some 802.11n point-to-point bridges top out at about 70 Mbps, which is the most that can be achieved in the absence of multipath. The ZoneFlex 7731 integrates a state-of-the-art, dual-polarized smart antenna. Because the antenna contains two, cross-polarized elements, the 7731 can achieve at least two spatial streams, even in the absence of multipath reflections. Connect802 has field-tested this bridge at 170 Mbps true TCP/IP throughput over a distance of about 1600 feet. We consider this to be a remarkable achievement for a radio in this price range. Please contact Connect802 Sales at 925.552.0802 to learn more about the Ruckus 7731 and other Ruckus products. We look forward to hearing from you!

Essential Wi-Fi

Overview of the new 802.3at Power-over-Ethernet Specifications
In the Fall 2009 issue, we discussed 802.3af, the IEEE standard that defines Power Over Ethernet (PoE). One of the limitations of 802.3af is that it can only provide up to about 15 W of power to each device. This limits the type of device that can be powered by 802.3af. It’s common for devices like 802.11n radios and motorized video cameras to require more power than 802.3af can provide. One solution to this problem has been proprietary, high-powered PoE technologies. These work, but have all the downsides that are usually associated with proprietary technologies. The standards-based solution is IEEE 802.3at, also known as PoE Plus.

The main thing that 802.3at offers over 802.3af is increased power, up to 25 watts per Powered Device (PD). In addition, the new standard redefines a PD as the PoE interface, instead of the device itself. Remember that 802.3af uses only half of the pairs in an Ethernet cable to carry power. With 802.3at, manufacturers could leverage this redefinition to put two power interfaces into a single device—one interface on one half of the pairs in the cable, and the other interface on the other half. The effect would be that the device could pull a total of up to 50 watts on a single Ethernet cable. Currently, Microsemi is the only manufacturer that we know of to be doing this. Although this technology is rare today, it’s great that this amount of power can be delivered while remaining complaint with the IEEE standard.

In addition to increased power, 802.3at offers more precision in power allocation. With 802.3af, power could be allocated in one of three levels. With 802.3at, power can be allocated in tenth-of-a-watt increments. This means that the power sourcing equipment can allocate only as much power to a device as the device is actually going to use, instead of having to “round up” to the next largest increment. The result is that more devices can be powered off of a single PSE, and fewer PSEs need to be purchased.

The issue of heat is an interesting side-note to the discussion of 802.3at. Copper cables have fairly high capability to carry electrical current, but heating of the cable can be an issue, especially when the cables are pulled in large bundles. Nobody wants to see a building burned down because PoE caused the Ethernet cable to catch fire! Fortunately, the IEEE took heating into consideration, and the standard has plenty of margin build in to ensure that, even if all pairs are powered in a large bundle, heating will not cause a safety issue. However, one concession to safety is that 802.3at requires the use of Cat-5e cable or better. Cat-5 cable or below can only be used with regular PoE, 802.3af, so if you’re going to use PoE Plus, make sure your cables make the grade.

 Technology and Engineering

Understanding the Free Space Path Loss Equation The electromagnetic field that expands outwards from a dipole antenna expands in a spherical manner. It's true that the shape of the 3dB volume is toroidal (doughnut shaped) but the surface of the volume grows in size as a function of the radius in a manner that is similar to that of an expanding sphere. For a sphere, the surface area is equal to four Pi times the square of the radius. When a sphere expands to twice it's volume the surface area increases by a factor of four. As seen on the right, the size of a particular area on the surface "stretches" and becomes larger (as the square of the increase in radius) as the volume expands. Some particular amount of energy is present in this volume and, hence, in this area of the expanding wavefront. As the wavefront expands and its area increases the density of energy in each one square centimeter of the wavefront surface decreases. It decreases because the surface of the wavefront is being spread out as the square of the increase of the radius. That's the "Inverse Square Law"   When we say that the strength of a signal decreases as the square of the distance we are saying that "If you're twice as far away the signal is 1/4 as strong" - the Inverse Square Law. The reason the signal is measured as being 1/4 as strong is that each square centimeter of area that is striking the capturing surface of the receiving antenna is now only receiving 1/4 as much energy (because the surface of the expanding wavefront has increased in area). Free space does not impede the propagation of electromagnetic signals in relationship to distance. The degree to which an electromagnetic wave is impeded across a 1-foot distance is the same as across a 1-mile distance. This is called the "impedance of free space and it's a constant, equal to 377 ohms.) The other factor that comes into play in the conventional "Free Space Path Loss" equation is the fact that an antenna designed to receive a higher frequency is smaller than one designed to receive a lower frequency. A 3 dBi antenna in the 2.4 GHz Wi-Fi band has an effective capture area of roughly 4 square inches. A 3 dBi antenna designed for the 5 GHz Wi-Fi band will have a capture area of only 1 square inch. This is because the wavelength is shorter at the higher frequency and the size of the antenna must be adjusted to match the wavelength (actually, 1/4 or 1/2 wavelength is typical). Because a higher frequency antenna has a smaller capture area (effective aperture) than a lower frequency antenna of the same dBi gain the higher frequency antenna will be unable to capture as much energy. This is the reason that one may hear that "higher frequencies are more attenuated than lower frequencies" - they're not. Higher frequencies are "attenuated" essentially identically to lower frequencies (within some boundaries). The reason that a 100 mW 802.11g 2.4 GHz access point is measured as having a -40 dBm RSSI at some location and a 100 mW 802.11a transmitter might measure -91 dBm at that same location is due to the difference in effective aperture (the difference in the capture area).

To Infinity... and Beyond!

Joe Bardwell's Podcast Interview with Network World

Network World presents Ron Nutter's HelpDesk Toolchest interview with Joe Bardwell, "Getting Ready for 802.11n: What You Need to Know" Listen to This Informative Podcast Now

Thinking about mode N (802.11n)? Want to know a little more about it before turning it up? In this podcast, Ron talks with Joe Bardwell, President and Chief Scientist with Connect 802 about what to be thinking about now that Mode N has been ratified. Joe will discuss the improvements that come with Mode N, 40 Mhz channels, OFDM and MIMO, among other topics.

4X4 MIMO Is Coming Soon
Although the 802.11n specifications were finalized in September 2009 it's only been recently that manufacturers have begun to announce 4X4 MIMO products. The Draft 2.0 802.11n standards, implemented by all 802.11n manufacturers today, support two spatial streams of simultaneous data ("2X2 MIMO"). 2X2 MIMO takes a 40 MHz 150 Mbps connection rate and doubles it to 300 Mbps. Although some manufacturers offer "3X3 MIMO" there are no notebook computers that can connect with more than two spatial streams so the "3X3" adds no speed advantage. 3X3 MIMO does, however, provide three alternative radios from which two spatial streams can be acquired - sort of "diversity antennas" for 802.11n.

A Google search for "4X4 MIMO" will reveal that some manufacturers are starting to offer a fourth radio with the capability of connecting using four spatial streams. That's what gets 802.11n to its maximum 600 Mbps connection rate. Unfortunately, until notebook computer, iPhones and other handheld devices support more than two spatial streams the benefits of 4X4 MIMO will be limited.

Web Searching: The Connect802 Web Presence

At Connect802 we're your PAGE ONE resource for wireless networking!

Connect802 has the experience, expertise, and resources to help you with your wireless network system. Use your favorite search engine and see what Connect802 is doing. Each month we give you some suggested search terms for you to explore. Here's this month's list. As you look down the search engine results you'll find Connect802 either at the top, or on the first page (true for Google and Excite, unknown for the rest).